The spine is formed of a column of vertebra that extends between the cranium and pelvis. The three major sections of the spine are known as the cervical, thoracic and lumbar regions. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae, with each of the 24 vertebrae being separated from each other by an intervertebral disc. A series of about 9 fused vertebrae extend from the lumbar region of the spine and make up the sacral and coccygeal regions of the vertebral column.
The main functions of the spine are to provide skeletal support and protect the spinal cord. Even slight disruptions to either the intervertebral discs or vertebrae can result in serious discomfort due to compression of nerve fibers either within the spinal cord or extending from the spinal cord. Disruptions can be caused by any number factors including normal degeneration that comes with age, trauma, or various medical conditions. If a disruption to the spine becomes severe enough, damage to a nerve or part of the spinal cord may occur and can result in partial to total loss of bodily functions (e.g., walking, talking, breathing, etc.). Therefore, it is of great interest and concern to be able to treat and correct ailments of the spine.
When conservative efforts fail, treating spinal ailments very often includes a combination of spinal fusion and fixation. Generally, spinal fusion procedures involve removing some or all of an intervertebral disc, and inserting one or more intervertebral implants into the resulting disc space. Introducing the intervertebral implant serves to restore the height between adjacent vertebrae (“disc height”) and maintain the height, and/or correct vertebral alignment issues, until bone growth across the disc space connects the adjacent vertebral bodies. During such procedures resection of ligaments and/or boney elements from the affected spinal area is common in order to access the disc space and/or decompress impinged nerve or spinal cord tissue. Though generally necessary to achieve the aims of the surgery, the resection of ligaments and/or boney tissue along the spine introduces instability (or, oftentimes, increased instability) to the spine.
Fixation systems are often surgically implanted during a fusion procedure to help stabilize the vertebrae to be fused until the fusion is complete or to address instabilities (either preexisting or created by the fusion or decompression procedure itself). Fixation constructs of various forms are well known in the art. Fixation systems usually use a combination of rods, plates, pedicle screws, and bone hooks to create a fixation construct across affected vertebrae. These fixations systems are designed to engage either the posterior elements (e.g. pedicle screw systems, spinous process plates) or anteriorly, the vertebral bodies (e.g. plates, anterior staple/rod systems). The configuration required for each procedure and patient varies due to the ailment being treated, the specific method of treatment (e.g. surgical approach, etc. . . .) and the patient's specific anatomical characteristics. Like the fusion, the fixation system can be implanted across a single level or across multiple levels, and typically, the fixation system is positioned to span at least each level to be fused. In severe cases the fixation construct may stretch along the majority of the spine.
Despite the tremendous benefits gained by patients (e.g. a reduction or elimination of symptoms such as pain, poor posture, etc. . . . ) which can be credited to the fusion/fixation procedures, the procedures are not without disadvantages. For example, the loss of motion at one or more levels of the spine increases the loads placed on remaining untreated levels. These increased loads can hasten a breakdown at nearby untreated levels (commonly referred to as adjacent level disease), or, cause a hardware failure in which a portion of the spinal fixation construct breaks, generally leading to a failed fusion and instability. The importance of spinal balance as a determinant factor for positive surgical outcomes (those that avoid or limit the effects just described and result in a positive reduction of symptoms) is increasingly being recognized. Spinopelvic measurements have been identified as critical parameters to consider when evaluating overall balance. Several studies correlate worsening HRQL (Health Related Quality of Living) parameters with positive postoperative sagittal balance (defined as SVA>5 cm, PT>20°, PI≠LL±9°), where SVA=sagittal vertical axis, PT=pelvic tilt, and PI=pelvic incidence. Other relevant anatomical measurements include K=thoracic kyphosis, LL=lumbar lordosis, SA=sagittal alignment, CA=coronal alignment, T1-tilt. Tools to help the surgeon assess intraoperative changes in overall balance however are lacking. The tools and methods set forth herein are directed towards addressing these challenges.